THE MEASUREMENT OF TEMPERATURE 51 



and forth with changes in temperature and so move the pointer. 

 The thermostat so often used in regulating the temperature of 

 buildings makes use of a thermometer of this kind. As the 

 temperature changes, the moving tip opens or 

 closes an electric circuit which operates the 

 dampers of the heating system. When the pointer 

 carries a pen and makes a continuous record on a 

 moving strip of paper, it is called a thermograph. 

 The indicator of the thermograph may also be 

 moved by the expansion of alcohol in a flattened 

 and curved metal tube. When the alcohol ex- 

 pands, the tube is straightened out and this moves 

 the pointer. The maximum thermometer is de- 

 signed to indicate the highest temperature to which 

 it has been exposed during a given period. In 

 such an instrument, a constriction just above the 

 bulb allows the mercury to squeeze through but 

 will not allow it to flow back again. The clinic 

 thermometer used by all physicians is a maximum 

 thermometer. Such instruments are set by shak- 

 ing the mercury down into the bulb again. The 

 minimum thermometer is an alcohol thermometer 

 having a small indicator in the tube which is 

 drawn down as the alcohol recedes but is not 

 pushed upward when the alcohol rises. The 

 minimum thermometer must be placed in a nearly 

 horizontal position. Its index may be set with a 



magnet or by giving the instrument a slight jar. clinical ther- 

 mometer. 

 Practical Exercises (Duff.) 



1. Examine the nearest thermometer. Is it made with mercury or 

 alcohol? 



2. How many Centigrade degrees equal 45 Fahrenheit degrees? 



